The present invention relates to the field of transistors, and more specifically to the field of metal-insulator-semiconductor field effect transistors formed on diamond substrates.
Transistors are solid state devices which amplify electrical signals and, when combined in large numbers close together on a single substrate, make up integrated circuits. Integrated circuits are employed in a wide variety of electronic applications, as for example, in microprocessors and computers.
Transistors are generally manufactured from wafers of single crystal silicon. However, the speed of silicon transistors is limited by the modest speed with which electrons flow within silicon conductive layers, and the lack of a sufficiently insulating form of silicon with which to produce low capacitance interconnections between devices. The speed of a transistor refers to its switching time. The speed limitations of silicon based transistors have been overcome by fabricating transistors from gallium arsenide. However, a gallium arsenide transistor is limited in its ability to operate at temperatures higher than about 450 K.
Diamond is a material with properties which theoretically allow transistor operation not only at higher speeds than gallium arsenide, but also at very high temperatures (perhaps as high as 900 K.). High temperature operation would be extend electronics into extreme thermal environments such as the inside of an operating airplane engine, deep within the earth in an oil drill hole, or in space vehicles operating in outer space without any cooling capability.
The first transistor behavior in diamond was demonstrated by J. F. Prins in 1982 [See J. F. Prins, "Bipolar Transistor Action In Ion Implanted Diamond," Appl. Phys. Lett., Vol. 41, p. 950, 1982.]The transistor developed by Prins was a so-called bipolar transistor which depended for its operation on the creation of adjacent regions in which electrons carried a majority of the current (called n-type regions) and holes carried a majority of the current (called p-type regions). In the Prins transistor, the p-type region was realized by starting with a substrate of naturally occurring diamond, called type IIb, a p-type material. The n-type region was made by directing an ion beam to impinge the surface of IIb diamond crystal to implant positive ions in the substrate in a process called ion implantation. Although the current-voltage characteristics of the Prins diamond transistor were typical of a bipolar transistor, the gain of the device was not significant.
The first diamond transistor with significant gain was a so-called point contact transistor made by M. W. Geis, et al, in 1987 [See M. W. Geis, D. D. Rathman, D. J. Ehrlich, R. A. Murphy, and W. T. Lindley, "High-Temperature Point-Contact Transistors And Schottky Diodes Formed On Synthetic Boron-Doped Diamond," IEEE Electron Device Lett., Vol. EDL-8, p. 341, 1987.]To make this transistor, Geis pressed tungsten points against the surface of a synthetic boron doped diamond crystal, which became a p-type semiconductor. Electron emission and collection regions were formed under each point. An advantage of the Geis transistor is that it provides a small signal current gain, which is defined as the ratio of change in output current to change in input current. At room temperature, the small signal current gain was about 20. The current gain decreased to about 2 at an operating temperature of 783 K. However, a principal disadvantage of the point contact transistor demonstrated by Geis is that it can not easily be reduced in size and combined with similar transistors to make a integrated circuit.
In 1991, reports began to appear in the open literature of planar diamond transistors which operated through the field effect. [See W. Tsai, M. Delfino, D. Hodul, R. Riaziat, L. Y. Ching, G. Reynolds, and C. B. Cooper, III, "Diamond MESFET Using Ultrashallow RTP Boron Doping," IEEE Electron Device Lett., Vol. EDL-2, p. 157, 1991; G. Sh. Gildenblat, S. A. Grot, C. W. Hatfield, and A. R. Badzian, "High-Temperature Thin-Film Diamond Field-Effect Transistor Fabricated Using A Selective Growth Method," IEEE Electron Device Lett., Vol. 12, p. 37, 1991.]Such transistors are called field effect transistors. "A field effect transistor is a transistor in which current carriers (holes or electrons) are injected at one terminal (the source) and pass to another (the drain) through a channel of semiconductor material whose resistivity depends mainly on the extent to which it is penetrated by a depletion region. The depletion region is produced by surrounding the channel with semiconductor material of the opposite conductivity and reverse-biasing the resulting p-n junction from a control terminal (the gate). The depth of the depletion region depends on the magnitude of the reverse bias." [Graf, r.f., Modern Dictionary of Electronics, Howard W. Sams & Co., Inc., p. 272, 1977.]
Field effect transistors are made by creating conducting layer near the surface of a semiconductor single crystal. The conducting layer is called a channel. For highest speed the semiconductor should be semi-insulating, that is, it should have a bulk resistivity of more than 10.sup.6 or 10.sup.7 ohm-cm. For high temperature operation the semiconductor should have a band gap of about 2 electron volts or more.
After creation of the conducting layer, ohmic electrical contact is made to the channel by forming two electrodes on the surface of the conducting layer. One electrode is called the source contact and one electrode is called the drain contact. A third electrode, referred to as a gate electrode, may be formed by depositing a metal directly on the channel, to create a metal-semiconductor field effect transistor. For a variety of reasons, including work function differences between the gate metal and semiconductor substrate, there is generally an electric field underneath the gate extending part way into the channel. The phrase "work function" refers to the amount of energy required to raise an electron from the Fermi level to the vacuum level. The field polarity is chosen in such a way that the flow of charge carriers in the channel is controlled in the region under the gate just as the flow of water is controlled by adjusting a valve in a water circuit. By changing the voltage on the gate electrode, the field in the channel can be changed causing the amount of current flowing inside the channel from source to drain to also be changed. A figure of merit of the transistor 10 of the present invention device is the ratio of the change in channel current to a change in gate voltage, called the transconductance, g.sub.m, of the transistor. The dimensions of transconductance are current/voltage, or Siemens.
By applying appropriate voltage at the source, drain, and gate electrodes, an electrical current may be induced to flow through the channel, entering through the source and leaving through the drain in much the same way water can be made to flow in one end and out the other end of a pipe.
W. Tsai et al. has made an FET on a diamond substrate having a transconductance of 1.6 .mu.S for each mm of gate width in accordance with the method described above. The channel in the Tsai transistor was made by placing a solid disk of boron nitride next to a piece of natural semi-insulating diamond and rapidly raising the temperature of both for time just sufficient for the boron ions to diffuse into the diamond and change, or "dope," the ion implanted region of the diamond into a p-type structure. One problem with Tsai's transistor was that it never achieved what is called current saturation, that is, it was never able to conduct a channel current as large as the channel was capable of handling. Because of this, the transconductance of the Tsai transistor was lower than it might otherwise have been. Saturation is required for low output conductance. Output conductance is the ratio of the change in channel current to the change in channel voltage where these changes are measured while keeping the gate voltage constant. Because the voltage gain of a transistor driving a large impedance load is given by the ratio of transconductance to output conductance, lack of saturation furthermore resulted in low voltage gain from the Tsai transistor.
As an alternative to forming a metal gate electrode directly on the channel, a gate electrode for a field effect transistor may also be manufactured by first depositing or growing an insulator, such as silicon dioxide, over the channel and then placing the gate electrode on top of the insulator. This device is called a metal-insulator-semiconductor field effect transistor or "MISFET." G. Sh. Gildenblat, et al., describes growing a channel directly on a diamond substrate using the well known technique of homoepitaxy. Channel growth was selective, that is, arrangements were made to grow the channel over certain parts of the substrate rather than over the entire surface of the substrate (which would have been called non-selective growth). The Gildenblat device operated at 573 K. and had a transconductance per unit gate width of 30 .mu.S mm.sup.-1. However, the Gildenblat device did not exhibit strong evidence of saturation in its transistor behavior, nor did it show any evidence of pinch-off, i.e., the ability to significantly reduce the channel current towards zero.